It is well known in the art that complexes of nickel with phosphorus-containing ligands are useful as catalysts in hydrocyanation reactions. Such nickel complexes using monodentate phosphites are known to catalyze hydrocyanation of butadiene to produce a mixture of pentenenitriles. These catalysts are also useful in the subsequent hydrocyanation of pentenenitriles to produce adiponitrile, an important intermediate in the production of nylon. It is further known that bidentate phoshite and phosphinite ligands can be used to form nickel-based catalysts to perform such hydrocyanation reactions.
U.S. Pat. No. 3,903,120 discloses a process for preparing zerovalent nickel complexes by reacting elemental nickel with a monodentate phosphorous ligand of the formula PZ3 where Z is an alkyl or alkoxy group, preferably an aryloxy group. The process uses finely divided elemental nickel and is preferably carried out in the presence of a nitrile solvent. The reaction is taught to be carried out in the presence of excess ligand.
U.S. Pat. No. 3,846,461 discloses a process for preparing zerovalent nickel complexes of triorganophosphites by reacting triorganophosphite compounds with nickel chloride in the presence of a finely divided reducing metal which is more electropositive than nickel, and in the presence of a promoter selected from the group consisting of NH3, NH4X, Zn(NH3)2X2, and mixtures of NH4X and ZnX2, where X is a halide. Reducing metals include Na, Li, Mg, Ca, Ba, Sr, Ti, V, Fe, Co, Cu, Zn, Cd, Al, Ga, In, Sn, Pb, and Th, with Zn being preferred.
U.S. Pat. No. 5,523,453 discloses a method of preparing nickel hydrocyanation catalysts containing bidentate phosphorous ligands. Zero-valent nickel compounds that contain ligands that can be displaced by the bidentate phosphorous ligand are a preferred source of nickel. Two such compounds are Ni(COD)2, where COD is 1,5-cyclooctadiene, and (oTTP)2Ni(C2H4), where oTTP is P(O-ortho-C6H4CH3)3. Alternatively, divalent nickel compounds may be combined with reducing agents to produce a suitable nickel source. In the latter method of preparing catalyst, as the temperature of the catalyst preparation increases, the catalyst formation rate increases, but the amount of degradation product also increases.
U.S. Pat. No. 6,069,267 describes a method for preparing a crude bidentate phosphorus-containing ligand suitable for use in the catalyst preparation. The resulting product is, however, not pure ligand, but rather a crude ligand mixture that contains byproducts of the reaction which may affect the rate of formation of the nickel-containing catalyst. That process does not provide for isolation and purification of the bidentate ligand. The present inventors observed that, compared to the use of purified ligand, such crude ligand mixtures do inhibit catalyst preparation reactions wherein divalent nickel compounds are contacted with reducing agents to produce the nickel catalyst.
Accordingly, in order to avoid costly purification of the ligand, there is a need in the art for a process for nickel catalyst preparation that counteracts the inhibiting effects of the impurities contained in the crude ligand.